1. Field of the Invention
The present invention relates to tunable filters including a plurality of elastic wave resonators, and particularly relates to a ladder circuit tunable filter.
2. Description of the Related Art
Many tunable filters capable of varying a pass band have been proposed thus far. For example, in the tunable filter disclosed in International Publication No. WO 2011/093449 A1, a serial arm resonator is disposed in a serial arm that connects an input end to an output end. An inductance for increasing the band width is connected in series to this serial arm resonator.
As another configuration, International Publication No. WO 2011/093449 A1 also discloses a ladder circuit tunable filter having a plurality of serial arm resonators and parallel arm resonators. According to this ladder circuit tunable filter, variable capacitors are connected in series and in parallel to the serial arm resonators.
Japanese Unexamined Patent Application Publication No. 2009-130831 also discloses a ladder circuit tunable filter. In Japanese Unexamined Patent Application Publication No. 2009-130831, respective variable capacitors are connected in series and in parallel to the serial arm resonators. Likewise, respective variable capacitors are connected in series and in parallel to the parallel arm resonators.
With the ladder circuit tunable filter disclosed in International Publication No. WO 2011/093449 A1, the ladder circuit tunable filter disclosed in Japanese Unexamined Patent Application Publication No. 2009-130831, and so on, connecting the variable capacitor not only enables the pass band to be varied, but also makes it possible to adjust a frequency-variable band width. However, the band of the resonator could not be sufficiently increased simply by connecting a variable capacitor.
Incidentally, with a ladder filter, an increase in out-of-band attenuation is achieved by increasing the ratio between serial arm resonator impedance and parallel arm resonator impedance. However, if the serial arm resonator impedance is too high, the filter will have increased insertion loss. Accordingly, the ratio between the serial arm resonator impedance and the parallel arm resonator impedance is normally set to three to six times.
Meanwhile, the tunable filter according to International Publication No. WO 2011/093449 A1 achieves an increase in band width by connecting an inductance to the serial arm resonator in series. However, there is a problem in that in the case where an attempt is made to widen the band of a serial arm or parallel arm resonator by connecting an inductance to the resonator, as in International Publication No. WO 2011/093449 A1, in a resonator where the ratio of serial arm resonator impedance and parallel arm resonator impedance is set to three to six times, the impedance ratio will increase even more. As a result, a resonant resistance of the serial resonator will increase, leading to a degradation in insertion loss and an inability to obtain sufficient characteristics.